Apparatus for forming a fiber optic coupler by dynamically adjusting pulling speed and heat intensity

ABSTRACT

Fused biconical tapered couplers formed from the pulling of heated optical fibers have controllable coupling ratios by dynamically adjusting the heat intensity and pull speed during fabrication. Fiber coupler fabrication begins by arranging two optical fibers, heating the optical fibers using a heat source a predetermined distance from the optical fibers, and pulling the optical fibers at an initial pulling velocity as the heated optical fibers soften. The heat source is moved away from the optical fibers and the pulling velocity is selectively reduced in response to a substantial change in the detected coupling ratio. Changes in the coupling ratio are controlled by selectively reducing the pulling speed and the heat intensity to a point where the pulling of the optical fibers may be halted, and the heat removed, without a substantial change in the detected coupling ratio. The coupling ratio may be monitored at different wavelengths for fabrication of wideband couplers and wavelength division multiplexers.

This application is a division of application Ser. No. 08/718,727 filedSep. 24, 1996 now abandoned.

FIELD OF THE INVENTION

The present invention relates to an arrangement for forming a fiberoptic device, for example, fused biconical tapered couplers.

DESCRIPTION OF THE RELATED ART

Fiber optic couplers are used as optical beam splitters between two ormore fibers, and are used in different optical systems, such as opticalcommunications. Fused biconical tapered couplers are formed generally byplacing two bare single mode fibers in contact with each other, addingtension to the fibers, and heating the fibers using a heat source, forexample a flame. As the fibers soften, they fuse together to form thefused biconical tapered coupler. The fused biconical tapered coupler mayalso be used as a wavelength division multiplexer, where opticalradiation at two or more wavelengths are combined through the coupler.

Considerable effort has been undertaken to control the opticalperformance of the fused biconical tapered coupler during manufacture.However, several variations during the heating process adversely affectthe final characteristics of the coupler. FIG. 1A is a diagramillustrating the coupling ratio 10 and 12 of an optical fiber at therespective wavelengths of 1550 nm and 1310 nm as the optical fibers arepulled during the fabrication process. As shown in FIG. 1A, the processbegins at the pull distance of 0 mm. The coupling ratio is 0% at 0 mm,indicating that all optical radiation transmitted from one optical fiberis received at the other end of that same optical fiber. Hence, there isno coupling of optical energy into the second optical fiber. At point10a, the optical radiation at 1550 nm has a 50% coupling ratio, wherehalf of the transmitted power is transmitted to each of the opticalfibers. At point 10b, the coupling ratio is 100%, where all of thetransmitted power is coupled to the second optical fiber. After reachingthe maximum coupling ratio at 10b, the coupling ratio for the opticalenergy at 1550 nm returns toward 50% at point 10c. As shown in FIG. 1A,the coupling ratio continues to oscillate between 0 and 100% as theoptical fibers are pulled.

Similarly, as the optical fibers are heated and pulled, the couplingratio 12 for 1310 nm increases to a point 12a intersecting with thecoupling ratio 10 of the first wavelength. The coupling ratio 12continues to increase to a maximum level of 100% at point 12b, and thencontinues to oscillate in its values as the optical fibers are pulledfurther.

As shown in FIG. 1A, it is desirable in the formation of a widebandcoupler to halt the change in coupling ratios such that the fiber opticcoupler has a stable coupling ratio corresponding to the points 10c and12a. However, the coupling ratio in FIG. 1A is generated by heating andpulling the optical fibers using a constant temperature and draw speed(i.e., pull speed). If during the pull process the heat source issuddenly removed, the value of the coupling ratio may jump from 1% to10%, preventing an operator from precisely setting the coupling ratio.Specifically, as shown in FIG. 1B, the coupling ratio 14 at a selectedwavelength increases as the optical fibers are pulled a certaindistance. Assuming that a 50% optical fiber coupler is to bemanufactured, the sudden removal of the heat source at the 50% pointwould actually cause the coupling ratio to jump to point 16, which mayhave a coupling ratio value of 75%. Constant heating of the opticalfibers creates a heated atmosphere around the coupler having a differentindex of refraction than cool air. In addition, the heated opticalfibers may have a different index of refraction than cooled opticalfibers. Hence, sudden removal of the flame causes a substantial changein the atmospheric index of refraction and a temperature change in theoptical fiber.

Hence, active monitoring of the coupling ratio during drawing results ininaccurate readings. If a technician desires to manufacture a 50%coupler, the technician will typically remove the flame at an earlierdetection level, such as 25% or 30%, in an effort to anticipate the jumpin the coupling ratio. However, such efforts in anticipating the jump inthe coupling ratio is determined by trial and error. Variables such asroom temperature, flame temperature, humidity, core and claddingthickness, etc., also affect the coupling ratio, and attempts to removethe flame at an earlier detection level may result in a jump in thecoupling ratio to the point 18, which may be outside the specificationrange for the fiber optic coupler.

Hence, fabrication of a fiber optic coupler is heavily dependent uponexperience of an operator in subjectively estimating the point at whichthe heat source should be removed in order to anticipate the jump in thecoupling ratio. The known coupler fabrication techniques are thusinherently unreliable, and do not provide a repeatable system forforming an optical fiber coupler having a desired coupling ratio.

DISCLOSURE OF THE INVENTION

There is a need for an arrangement for consistently forming a fiberoptic coupler having a desired coupling ratio for a selected wavelength.

There is also a need for an arrangement for forming a fiber opticcoupler, where changes in the coupling ratio during coupler formationcan be consistently controlled.

There is also a need for an arrangement for forming a fiber opticcoupler that selectively changes the rate of change in a detectedcoupling ratio during formation of the coupler.

These and other needs are attained by the present invention, where thepulling speed and heat intensity are dynamically controlled inaccordance with the detected coupling ratio to control the rate ofchange in the coupling ratio.

According to one aspect of the present invention, a method of forming afiber optic device from first and second optical fibers contacting eachother at a fusion region includes the steps of applying incident heat onthe first and second optical fibers at the fusion region with a heatsource positioned at a predetermined distance from the optical fibers,pulling the heated first and second optical fibers at a pullingvelocity, reducing the incident heat on the first and second opticalfibers at a first reduction rate in response to a change in a detectedcoupling ratio between the first and second optical fibers, adjustingthe pulling velocity by a first acceleration rate in response to thechange in the detected coupling ratio, and repeating the reducing andadjusting steps in accordance with the detected coupling ratio. Changingthe incident heat and the pulling velocity of the optical fibers inaccordance with the detected coupling ratio maintains control over thechange in the detected coupling ratio. Hence, the incident heat and thepulling velocity may be adjusted to a point that subsequent changes tothe incident heat or pulling velocity have little or no effect on thefinal coupling ratio, resulting in a system that maintains control overthe final coupling ratio of the formed fiber optic coupler.

According to another aspect of the present invention, a method offorming a fiber optic coupler from first and second optical fiberscontacting each other at a fusion region comprises the steps of applyingincident heat on the first and second optical fibers at the fusionregion with a heat source positioned a predetermined distance from theoptical fibers, pulling the heated first and second optical fibers at afirst velocity, reducing the incident heat on the first and secondoptical fibers at a first reduction rate in response to a change in adetected coupling ratio between the first and second optical fibers,adjusting the first velocity by a first acceleration rate in response tothe change in the detected coupling ratio, and selectively changing atleast one of the first reduction rate and the first acceleration rate inaccordance with the detected coupling ratio. Selectively changing thefirst reduction rate and/or the first acceleration rate ensures controlover the coupling ratio and the rate of change of the coupling ratioduring formation of the optical fiber coupler. Hence, if the couplingratio is changing at an undesired rate, the first reduction rate and/orfirst acceleration rate may be changed to provide a more desirable rateof change in the detected coupling ratio. Hence, the manufacture offiber optic couplers can be precisely controlled to form couplers havingaccurate coupling ratios.

Still another aspect of the present invention provides an apparatus forfusing first and second optical fibers each having a first end secured,the apparatus comprising at least one actuator pulling a second end ofthe first and second optical fibers at a pulling velocity, at least onedetector calculating a coupling ratio between the first and secondoptical fibers in response to detected optical energy output at an endof at least one of the optical fibers, a heat source applying heat tothe first and second optical fibers at a selected intensity, and acontroller controlling the pulling velocity and the heat intensity, thecontroller successively reducing the pulling velocity and the heatintensity at the respective first and second reduction rates inaccordance with the detected coupling ratio. Hence, the apparatus of thepresent invention dynamically adjusts the pulling velocity and heatintensity to maintain control over the detected coupling ratio. Hence,the present invention enables formation of a fiber optic coupler havinga desired coupling ratio with consistent results.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent like elements throughoutand wherein:

FIGS. 1A and 1B are diagrams illustrating the variation in couplingratio during conventional coupler formation techniques.

FIG. 2 is a block diagram of an apparatus for fusing first and secondoptical fibers to form a fiber optic coupler according to an embodimentof the present invention.

FIG. 3 is a flow diagram summarizing the method of forming a fiber opticcoupler according to an embodiment of the present invention.

FIGS. 4A and 4B are diagrams illustrating pulling velocity and flamemovement velocity according to the disclosed embodiment, respectively.

FIG. 5 is a diagram illustrating the orientation of the heat source ofFIG. 2 relative to the optical fibers.

FIGS. 6A and 6B are diagrams illustrating changes in the coupling ratioduring formation of the fiber optic coupler according to the disclosedembodiment.

FIGS. 7A and 7B are diagrams illustrating the changes in the couplingratio at different wavelengths during formation of a wideband couplerand wave division multiplexer, respectively, according to the disclosedembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The disclosed arrangement for forming a fiber optic coupler maintainscontrol of the coupling ratio by maintaining constant control of theheat intensity applied to the optical fibers, and the pull speed duringthe pulling of the optical fibers. Fiber coupler fabrication begins byarranging two optical fibers contacting each other within a fusionregion, heating the optical fibers, and pulling the optical fibers asthe heated optical fibers soften. Once a substantial change in thedetected coupling ratio is identified, the heating intensity and thepulling velocity are selectively reduced to maintain an accurate readingof the coupling ratio during formation of the optical coupler. Hence,maintaining control of the coupling ratio by selectively reducing thepulling speed and the heat intensity ensures an accurate reading of thecoupling ratio without an overshoot of the measured coupling ratio whenthe heating element is removed.

As described below, the incident heat and the pull velocity are reducedin response to preferably a substantial change in the detected couplingratio. The substantial change in the detected coupling ratio is definedas the point during the fusion process at which the two optical fibersbegin fusing, resulting in a coupling ratio change from 0% (i.e., alloptical power energy in one optical fiber) to 3%-5%. When the detectedcoupling ratio is in the range of 3% and 5%, the optical fibers arephysically fused, but are just beginning to optically couple transmittedoptical energy. This point is referred to as the initial coupling point.

The incident heat on the first and second optical fibers may be reducedafter the initial coupling point in view of the realization that alarger amount of heat is unnecessary to pull the optical fibers.Specifically, optical fibers are typically formed of fused quartz. Fusedquartz is a poor conductor of heat, and does not quickly dissipate heat.Hence, a relatively large amount of heat is initially required to softenthe fibers sufficiently to cause physical fusion. However, the softenedfibers tend to retain the heat during pulling. Consequently, once theoptical fibers have been heated to the initial coupling point, the heatmay be subsequently reduced to the minimal amount necessary to pull theoptical fibers. As the heat source intensity is reduced, the pull speedmay also be reduced to reduce the rate of change in the coupling ratio.The change in heat intensity itself does not change the coupling ratioper se. Rather, changes in the heat source intensity and the pull speedmay affect the shape of the taper being formed. Further, the shape ofthe taper determines the characteristics of the coupling ratio. Hence,the gradual reduction in heat intensity and pull speed enables thepulling process to be halted and the flame removed when the desiredcoupling ratio has been reached, without a sudden change in the measuredcoupling ratio.

FIG. 2 is a block diagram of an apparatus for fusing optical fibersaccording to an embodiment of the present invention. As shown on FIG. 2,the apparatus includes stages 20 and 22 that are movable along rails 24in an axis x parallel to the length of the optical fibers 28 and 30.Each stage 20 and 22 includes a mount 26 that secures optical fibers 28and 30 to the corresponding stage. The position of the stages 20 and 22are controlled by actuators 32 and 34, respectively. Each of theactuators 32 and 34 include stepper motors providing high resolution inthe translational movement of the stages 20 and 22.

The actuators 32 and 34 each pull the optical fibers in oppositedirections at a pull speed (1/2v_(p)) in response to control signalsoutput from a controller 36. Hence, the overall pull speed of theoptical fibers equals v_(p). The controller 36 monitors thedisplacement, velocity and acceleration of the stages 20 and 22 underthe control of the respective actuators 32 and 34. The actuators 32 and34 are designed to operate in a symmetric manner, where the stages 20and 22 move at the same velocity in opposite directions. Hence, thepulling velocity (v_(p)) equals two times the speed of each stage.

The apparatus of FIG. 2 also includes a laser 42 that outputs opticallaser energy toward a lens 44 that directs the optical energy into aninput end of the optical fiber 28. The corresponding end of the opticalfiber 30 can be left open, or it can be coupled to a third detector tomeasure back-reflected laser energy. A detector (D1) 38 monitors theoutput power of optical energy from the end of optical fiber 28, and adetector (D2) 40 monitors the output power of optical energy from theend of the optical fiber 30.

The apparatus of FIG. 2 further comprises a heat source 50 that appliesheat to the first and second optical fibers at a selected intensityunder the control of the controller 36. According to the disclosedembodiment, the heat source 50 is implemented as a flame sourceoutputting a flame having a predetermined height and positioned at apredetermined distance relative to the optical fibers 28 and 30. Theheat source 50 is controlled by an actuator 52 that controls theintensity of the applied heat by moving the heat source 50 away from theoptical fibers along the z direction. The actuator 52 moves the heatsource 50 at a flame velocity (v_(f)) controlled by the controller 36.

If desired, the flame source 50 may be substituted with an electric heatsource or a laser heat source. In such a case, the heat intensity may becontrolled either by moving the heat source away from the opticalfibers, or by controlling the power supplied to the electric heat sourceor laser heat source. The intensity of the laser heat source may also bereduced by diffusing the laser beam.

The controller 36 monitors the overall coupling operation and controlsthe actuators to ensure that the desired coupling ratio is achieved in acontrollable manner. Specifically, the controller 36 includes a controlportion 42 and a plurality of registers 44 that store operational datarelated to the coupling process. For example, the controller 36 monitorspulling velocity (v_(p)), the flame velocity (v_(f)) identifying thespeed of the heat source 50 along the z axis. The controller 36 alsomonitors the position of the heat source along the z direction as afunction of the distance from the optical fibers (d_(f)), and thepositions of the stage 20 (x_(s1)) and stage 22 (x_(s2)). As describedbelow, the controller 36 also stores a selected acceleration rate(a_(p)) for the pulling velocity of the stages and the acceleration(a_(f)) of the flame velocity.

The controller 36 includes a control portion 42 that calculates thecoupling ratio at selected wavelengths in response to the powergenerated by the detectors 38 and 40. For example, each detector outputsan output power identifying the detected optical energy output at an endof the corresponding optical fiber. The control portion 42 calculatesthe coupling ratio (CR) at the selected wavelengths, for example,L1=1310 nm, and L2=1550 nm. Alternately, the coupling ratio can bedetermined using a single one of the detectors. The control portion 42also calculates and monitors the rate of change of the coupling ratios(d(CR)/dt) by determining the differential in the coupling ratio perunit time, for example, one second.

As described below, the controller 36 controls the rate of change in thecoupling ratio (CR) by dynamically controlling the pulling velocity, andby controlling changes in the heat intensity, i.e., the position andspeed of the heat source. The controller 36 gradually adjusts thepulling speed and the heat intensity to ensure that any changes in thepulling velocity and the heat intensity do not cause unwanted changes inthe shape in the taper, which may otherwise cause a sudden change in thecoupling ratio at the selected wavelengths.

The actuator 52 also is adapted to move the heat source 50 axially alongthe optical fibers 28 and 30 within a fusion region 51 having a length,for example, of 15 mm. This axial movement, known as dithering,distributes the heat along the fusion region 51. As recognized in theart, the actuator 52 may be modified to provide variations on thetraveling path of the heat source 50 relative to the optical fibers toform more than one fusion area or modify the shape of the taper.

FIG. 3 is a flow diagram of the method of forming a fiber optic coupleraccording to an embodiment of the present invention. The method of FIG.3 is under the control of the controller 36, enabling implementation ofa fully automated system. The method begins in step 60 by initiallysetting up the apparatus of FIG. 2. The actuators 32 and 34 are presetsuch that the stages 20 and 22 are positioned closest to each other,i.e., the stage 20 is moved to the right-most position and the stage 22is moved to the left-most position on the rails 24. The optical fibers28 and 30 are then clamped within the mounts 26 of the stages 20 and 22with sufficient initial tension to straighten the optical fibers 28 and30, ensuring that the optical fibers contact each other at the fusionregion 51.

After the optical fibers 28 and 30 have been mounted on the stages 20and 22, the detectors 38 and 40 are coupled to the output ends of theoptical fibers 28 and 30, respectively. The detectors are thencalibrated using the laser 42 in step 62. In other words, the laser 42outputs the optical laser energy at a known wavelength and a known inputpower to the optical fiber 28. Hence, the detector 38 should detect 100%of the output power from the optical fiber 28, and the detector 40should detect 0% of the power at the output of the optical fiber 30. Ifthe detector 40 outputs a reading suggesting a detected output powerfrom the optical fiber 30, the controller 36 calibrates the detector 40to 0%.

After the detectors 38 and 40 are calibrated to the laser 42 and theoptical fibers 28 and 30 are positioned on the stages 20 and 22, thecoupler formation process begins in step 64 by applying heat to theoptical fibers 28 and 30. The heat is applied at a predetermined initialintensity by placing the flame generated by the heat source 50 at apredetermined distance (d_(f) =d₀) from the optical fibers, for exampleby placing the fibers at the tip of the flame.

Once the optical fibers have softened, the controller 36 starts pullingthe optical fibers in step 66 at an initial velocity of v_(p) =v₀, wherev₀ is equal to, for example, 5 mm per minute. The initial distance ofthe heat source 50 (d₀) and the initial pull speed (v₀) are maintaineduntil the controller 36 detects in step 68 a substantial change in thedetected coupling ratio of between 3% to 5%. The substantial change inthe coupling ratio from 0 to at least 3% identifies the transition pointat which the optical fibers begin to optically couple the transmittedlaser energy.

As described earlier, if the coupling ratio is allowed to increase bymaintaining a constant pull velocity and a constant heat intensity, thecoupling ratio will continue to increase in the manner shown in FIGS. 1Aand 1B. It has been discovered that the rate of increase in the couplingratio increases substantially after the coupling ratio reaches 10%.Hence, the control of the coupling ratio should begin before thecoupling ratio reaches 10%.

The control of the coupling ratio begins in step 70 by decreasing thepull speed at a specified deceleration rate a_(p) (t). The value ofa_(p) may be implemented as a percentage, for example, decreasing thepull speed by 4% per second. Alternately, the acceleration may becalculated as a continuous function, where the pulling velocity iscalculated according to the equation v_(p) (t)=v₀ (e)^(t1nx), where x isequal to 1 minus the percent decrease. Hence, if the pull speed is to bereduced by 4% per second, then x=0.96, and the equivalent pull equationwould be:

    v.sub.p (t)=v.sub.0 (e).sup.-0.04t

FIG. 4A is a diagram illustrating the reduction in pull speed withrespect to the pulled distance. As shown in FIG. 4A, the pull speed isconstant until the initial coupling point 96, at which point thecontroller 36 detects in step 68 a substantial change in the detectedcoupling ratio. The velocity curves 90, 92 and 94 correspond toacceleration rates of -2% per second, -4% per second, and -6% persecond, respectively.

The coupling ratio is then controlled in step 72 by moving the heatsource 50 away from the optical fibers at a speed of v_(f) (t) shown inFIG. 4B.

FIG. 5 is a diagram illustrating the relationship between the distanceof the heating element 50 from the optical fibers 28 and 30. As shown inFIG. 5, the distance (d_(f)) is varied during the coupler formationprocess from the initial distance (d₀), where the initial distance d₀has a value of approximately 0.5 mm. Hence, the heat source 50 ispositioned such that the optical fibers 28 and 30 are positioned withinthe flame output by the heat source 50. As recognized in the art, theinitial position of the heat source 50 is necessary to soften theoptical fibers 28 and 30 sufficient to overcome the surface tension ofthe optical fibers. Once the optical fibers begin to physically fuse,the incident heat on the optical fibers is reduced by moving the heatsource 50 away from the optical fibers. To prevent the movement of theheat source 50 from causing a change in the shape of the taper in thecoupler, the pull speed is simultaneously reduced such that the ratiobetween the heat intensity and the pull speed is substantially constant.Hence, reducing the flame reduces the rate of change in the couplingratio when the pull speed is reduced accordingly. Moreover, a reductionin heat intensity ensures the coupling ratio readings are not distorted.Hence, by reducing the heat intensity until the point where the opticalfibers are soft enough to be pulled, the method of forming the couplermay be maintained over long periods of time, for example, up to onehour, by slowly reducing the pull speed of the stages 20 and 22. Thus,additional time is available for measurement and testing using thedetectors 38 and 40.

Thus, the initial velocity of the heat source 50 before the initialcoupling point 96 is zero in order to provide the maximum amount of heatto the optical fibers at the initial distance (d_(f) =d₀). The heatsource 50 is then moved away in step 72 at a maximum initial speed, forexample, 5 mm per minute. FIG. 4B illustrates the velocity curves 98,100 and 102 for a speed reduction in the heat source of -1% per second,-2% per second, and -3% per second, respectively.

After the pull speed (v_(p)) and the heat intensity (d_(f)) have beenchanged in steps 70 and 72, respectively, the coupling ratio (CR) ischecked at the selected wavelengths L1 and L2. If desired, thecontroller 36 may also calculate in step 74 the rate of change in thecoupling ratio. After calculating the coupling ratio at the selectedwavelengths and the rate of change in the coupling ratio, the controller36 decides in step 76 whether the heat reduction rate (a_(f)) or thedeceleration (a_(p)) of the pull speed needs to be changed in step 78.If no change in the deceleration rates is needed, the controller 36returns to step 70 to selectively decrease the pull speed and the flamevelocity for the next iteration. According to the disclosed embodiment,the decrease in steps 70 and 72 are implemented in one seconditerations.

FIG. 6A is a diagram illustrating the change in the coupling ratioaccording to the method of the present invention. As shown in FIG. 6A, acoupling ratio curve 110 increases steadily until reaching a desiredlevel 112. As described above, the heat source 50 is preferablygradually reduced before reaching the desired coupling ratio 112 toensure any sudden changes in the heat intensity do not distort thecoupling ratio readings. Hence, the final incremental changes in thecoupling ratio along the curve 110 is accomplished by maintaining theheat source 50 at a reduced intensity sufficient to enable the opticalfibers to be pulled, steadily decreasing the pull speed (v_(p)) untilthe desired coupling ratio level 112 is reached, at which point the pullspeed (v_(p)) is set to zero (v_(p) =0) and the heat source 50 isremoved without causing a change in the coupling ratio.

The disclosed embodiment thus provides full control over the changes inthe coupling ratio by dynamically adjusting the heat intensity and thepulling velocity. In addition, once the heat intensity has been reducedby a sufficient amount, i.e., once the flame has been moved far enoughaway, accurate measurements of the coupling ratio may be obtained, suchthat the pulling process may be halted precisely at the desired couplingratio. Hence, the method of the present invention enables the heatingand pulling process to be abruptly halted without adversely affectingthe detected coupling ratio.

An additional feature of the disclosed embodiment is that the change inthe coupling ratio may be selectively increased or decreased toaccurately reach the desired coupling ratio 112. For example, FIG. 6Aillustrates a coupling ratio curve 110 having a generally constant rateof increase. FIG. 6A also illustrates coupling ratio curves 114 and 116having different rates of change in the coupling ratio. For example,assuming the coupling ratio curve 110 has a substantially constantincrease of 0.5% per second, the coupling curve 114 has a fast rateportion 118, followed by a slow rate portion 120. For example, the fastrate portion 118 corresponds to an increase of 1.5% per second, and theslow rate portion corresponds to a rate of 0.25% per second. The changein the coupling ratio curve 114 occurs at point 124, at which point theacceleration rate of the pull speed (a_(p)) and/or the acceleration rateof the flame velocity (a_(f)) is changed in step 78 of FIG. 3.Conversely, the curve 116 illustrates a slow rate 120, followed by afast rate 118 after the transition point 126.

Table 1 illustrates how the coupling ratio may be increased or decreasedby changing the pull rate or the flame rate.

                  TABLE 1                                                         ______________________________________                                                 Coupling                                                             Example  Ratio        Pull Rate                                                                              Flame Rate                                     ______________________________________                                        1        1.5%/sec     -2%/sec  -2%/sec                                        2        0.5%/sec     -4%/sec  -2%/sec                                        3        0.25%/sec    -6%/sec  -2%/sec                                        4        0.25%/sec    -4%/sec  -1%/sec                                        5        0.5%/sec     -4%/sec  -2%/sec                                        6        1.5%/sec     -4%/sec  -3%/sec                                        ______________________________________                                    

As shown in Examples 1, 2 and 3 of Table 1, the rate of change in thecoupling ratio may be increased by minimizing the decrease in the pullspeed. Hence, if the rate of change in heat intensity is constant, therate of change in the coupling ratio may be maximized by minimizing thedecrease in the pull rate. Similarly, Examples 4-6 illustrate the pullrate decreasing at a constant rate, with the flame rate having variousacceleration factors while in the coupling change region. As shown inTable 1, Example 4 has the slowest deceleration flame rate of -1% persecond and provides the slowest increase in coupling ratio of 0.25% persecond.

Hence, the rate of change in the coupling ratio can be controlled byselectively changing the acceleration rate (a_(p)) in the pullingvelocity, also referred to as the pull rate, and/or changing the flamerate (a_(f)) in step 78. Thus, the rate of change in the coupling ratiomay be reduced by maximizing the decrease in the pull rate whilemaintaining a constant rate of change in heat intensity.

Hence, the coupling ratio can be controlled as shown in FIG. 6A toprovide maximum accuracy in obtaining the desired coupling ratio 112. Asshown in curve 114, there may be more than one transition point inreaching the desired coupling ratio 112. For example, the curve 114begins at change rate 118, and then transitions at point 124 to thelower change rate 120. As the coupling ratio approaches the desiredcoupling ratio 112, a second transition point 124' further reduces therate of change in the coupling ratio. Although not desirable, the pullspeed and flame intensity can be increased, if necessary.

FIG. 6B illustrates different rate curves for a coupling ratio that haspassed the maximum 100% coupling ratio. In this case, the change in thecoupling ratio is modified at transition point 127 by selectivelychanging the acceleration rates in step 78 of FIG. 3A. Hence, assumingrate curve 130 corresponds to the velocity curves 92 and 100 of FIGS. 4Aand 4B, respectively, the rate curve 128 may be obtained by using atleast one of the velocity curves 94 and 102 of FIGS. 4A and 4B,respectively. Similarly, the rate curve 132 may be obtained by using atleast one of the velocity curves 90 and 98, respectively.

FIG. 7A is a diagram illustrating the coupling ratio rate curves for twoselected wavelengths in the formation of a wideband coupler according tothe disclosed embodiment. The rate curve 140 represents the couplingratio at 1550 nm, and the rate curve 142 represents the coupling ratioat 1310 nm. The rate curves 140 and 142 intersect at point 148. As shownin FIG. 7A, a wideband coupler having a coupling ratio between 45% and55% may be obtained by gradually adjusting the acceleration in step 78of FIG. 3A to reduce the change in the respective coupling ratios.Hence, by adjusting the acceleration rates in step 78, the respectivecoupling ratio curves are changed to curves 140' and 142' for 1550 nmand 1310 nm, respectively.

FIG. 7B is a diagram showing the coupling ratio curves for the twowavelengths 1550 nm and 1310 nm in forming a wave division multiplexeraccording to the disclosed embodiment. Hence, the control of thecoupling ratio enables more precise control in the formation of widebandcouplers and wave division multiplexers having precise coupling ratios.

According to the present invention, the incident heat on the opticalfibers and the pull velocity of the optical fibers are reduced inresponse to a substantial change in the detected coupling ratio betweenthe optical fibers. Dynamic adjustment of the heat intensity and thepull speed provides greater control over the shape of the taper andprovides more accurate reading of the coupling ratio, resulting information of optical couplers having reliable coupling ratios.

In addition, the above embodiment can be implemented to form an opticalattenuator, where an optical fiber is selectively tapered to provide adesired attenuation.

An additional feature of the disclosed embodiment is that no stressoptic effects are induced. Hence, the coupler formed according to thedisclosed embodiment does not have a high degree of polarization.Rather, the polarization of the coupler formed according to thedisclosed embodiment may be as low, and possible lower than aconventional coupler. Moreover, experiments have found the couplerformed according to the disclosed embodiment to have a lower loss thanconventional couplers, while at the same time providing greaterstructural strength. Hence, the arrangement for forming optical couplersaccording to the disclosed embodiment provides optical couplers havingreliable coupling ratios and optimum optical properties. The disclosedmethods are also applicable to single wavelength couplers, and multimodecouplers made from multimode fibers.

As described above, the heat source 50 may be moved axially along thelength of the optical fiber along a fusion region 51. This heatingtechnique, known as dithering, may be modified by moving the heat sourcein circular or figure-eight patterns to form two or three fusion areasor change the taper shape, as desired.

In addition, it will be recognized that the heat source 50, disclosed asa flame burner, may be substituted with an electric heater capable ofgenerating an operating temperature of 1573 K.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

I claim:
 1. An apparatus for fusing first and second optical fibers eachhaving a first and second ends, the apparatus comprising:at least oneactuator capable of pulling at least one of the first and second ends ofeach of the first and second optical fibers at a pulling velocity; atleast one detector capable of detecting a coupling ratio between thefirst and second optical fibers in response to detected optical energyoutput at the at least one of the first and second ends of each of thefirst and second optical fibers; a heat source capable of applying heatto the first and second optical fibers at a selected intensity; and acontroller responsively connected to said at least one detector, andreceiving the coupling ratio detected via said at least one detector andmonitoring and determining a rate of change of the coupling ratio, saidcontroller capable of separately controlling the pulling velocity andthe heat intensity in accordance with the rate of change of the detectedcoupling ratio.
 2. The apparatus of claim 1, wherein the controller iscapable of changing said pulling velocity from an initial pullingvelocity and said heat intensity from an initial heat intensity when asubstantial change in the rate of change of the detected coupling ratiois detected.
 3. The apparatus of claim 1, wherein the at least onedetector is capable of calculating the coupling ratio including firstand second optical wavelengths, and wherein the controller is capable ofselectively adjusting the pulling velocity and the heat intensity inaccordance with the calculated coupling ratio including the first andsecond optical wavelengths.
 4. The apparatus of claim 1, furthercomprising a second actuator, responsive to said controller, capable ofmoving said heat source away from the first and second optical fibers ata velocity corresponding to said adjusting of the heat intensity.
 5. Theapparatus of claim 4, wherein the controller is capable of the moving ofsaid heat source by said second actuator in response to detection of asubstantial change in the detected coupling ratio.
 6. The apparatus ofclaim 5, wherein said substantial change in the detected coupling ratiois within a range of between three and five percent.
 7. An apparatus forfusing first and second optical fibers each having a first and secondends, the apparatus comprising:at least one actuator capable of variablypulling at least one of the first and second ends of each of the firstand second optical fibers at a pulling velocity; at least one detectorcapable of detecting a coupling ratio between the first and secondoptical fibers in response to detected optical energy output at the atleast one of the first and second ends of each of the first and secondoptical fibers; a heat source capable of applying heat to the first andsecond optical fibers at a selected intensity; and a controllerresponsively connected to said at least one detector, and receiving thecoupling ratio detected via said at least one detector and monitoringand determining a rate of change of the coupling ratio, said controllercapable of separately dynamically controlling the pulling velocity andthe heat intensity in accordance with the rate of change of the detectedcoupling ratio throughout the fusion of the first and second opticalfibers.
 8. An apparatus for fusing first and second optical fibers eachhaving a first and second ends, the apparatus comprising:at least oneactuator capable of variably pulling at least one of the first andsecond ends of each of the first and second optical fibers at a pullingvelocity under a variable tension; at least one detector capable ofdetecting a coupling ratio between the first and second optical fibersin response to detected optical energy output at the at least one of thefirst and second ends of each of the first and second optical fibers; aheat source capable of applying heat to the first and second opticalfibers at a selected intensity; and a controller responsively connectedto said at least one detector, and receiving the coupling ratio detectedvia said at least one detector and monitoring and determining a rate ofchange of the coupling ratio, said controller capable of separatelydynamically controlling the pulling velocity and the heat intensity inaccordance with the rate of change of the detected coupling ratiothroughout the fusion of the first and second optical fibers.
 9. Anapparatus according to claim 8, wherein the change in coupling ratiocomprises at least a substantial change in the coupling ratio.
 10. Anapparatus for fusing first and second optical fibers each having a firstand second ends, the apparatus comprising:at least one actuator capableof variably pulling at least one of the first and second ends of each ofthe first and second optical fibers at a pulling velocity under avariable tension; at least one detector capable of detecting a couplingratio between the first and second optical fibers in response todetected optical energy output at the at least one of the first andsecond ends of each of the first and second optical fibers; a heatsource capable of applying heat to the first and second optical fibersat a selected intensity; and a controller responsively connected to saidat least one detector, and receiving the coupling ratio detected viasaid at least one detector and monitoring and determining a rate ofchange of the coupling ratio, said controller capable of separatelydynamically controlling the pulling velocity and the heat intensity inaccordance with the change of the detected coupling ratio throughout thefusion of the first and second optical fibers while the detectedcoupling ratio progresses from an initial coupling ratio substantiallyat the beginning of the fusing and a final coupling ratio substantiallyat the end of the fusing.